1. Field of the Invention
The present invention relates to a semi-transmissive (trans-reflective) type liquid-crystal display (LCD) device and a method of fabricating the same. More particularly, the invention relates to a semi-transmissive type LCD device having pixel regions each of which has a transmission electrode and a reflection electrode, where a barrier metal film is formed between the transmission electrode and the reflection electrode, and a method of fabricating the device.
2. Description of the Related Art
In recent years, subsequent to the diffusion of portable information-processing equipment, the semi-transmissive type LCD device has been developed and used practically. With the device of this type, in a place where the ambient light is insufficient (i.e., a dark place), images are displayed by causing the light emitted from the backlight unit to pass through the liquid crystal layer. On the other hand, in a place where the ambient light is sufficient (i.e., a light place), images are displayed by causing the ambient light reflected by the internal reflection electrodes to pass through the liquid crystal layer.
With the semi-transmissive type LCD device having such the structure as above, the backlight is turned on and the images are displayed in the transmission mode in a dark place, thereby raising the visibility. The backlight is turned off and the images are displayed in the reflection mode in a light place, thereby reducing the power dissipation. Because of such the advantage, this device meets the two inconsistent demands to prolong the operable time and to reduce the weight. Recently, the semi-transmissive type LCD device having such the features has been frequently used for portable information-processing equipment having a middle- or small-sized display screen.
The semi-transmissive type LCD device may have a structure that each of the pixel regions is divided into a transmission region and a reflection region. With this structure, a transmission electrode is placed in the transmission region and a reflection electrode is placed in the reflection region. In other words, each of the pixel electrodes is formed by the transmission electrode and the reflection electrode. The transmission electrode and the reflection electrode are electrically interconnected.
With the semi-transmissive type LCD device whose pixel region is divided into the transmission region and the reflection region, it is known that a problem termed “corrosion of an ITO film due to cell corrosion reaction” occurs because each pixel electrode is formed by the transmission electrode and the reflection electrode, as disclosed in the Patent Document 1 (Japanese Non-Examined Patent Publication No. 2004-144826) (see paragraph 0004 to 0020).
Specifically, it is general that the transmission electrode is formed by a patterned ITO (Indium Tin Oxide) film and the reflection electrode is formed by a patterned aluminum (Al) film. This is because the ITO film has a high transmittance for visible light and the Al film has a high reflectance for visible light. In addition, an Al alloy film may be used for the reflection electrode instead of the Al film; however, the action and advantages in the case where an Al alloy film is used are the same as those in the case where an Al film is used. Thus, the following explanation will be made for the case where the Al film is used for the reflection electrode.
By the way, in the fabrication process sequence of the TFT array substrate where thin-film transistors (TFTs) are arranged in a matrix array on an insulative substrate as the switching elements of the semi-transmissive type LCD device, a positive type photoresist is usually used. This is because a positive type photoresist realizes a higher resolution than that generated by a negative type photoresist. Since a developer solution with high alkalinity is used in the developing process of the positive type photoresist, the Al film, which is soluble in an acidic or alkaline solution, will dissolve into the developer solution during the developing process, where the Al atoms are turned to trivalent Al ions.
On the other hand, the ITO film is insoluble in the developer solution. However, the oxidation-reduction potential of the ITO film is nobler (in a positive side) than that of the Al film, in other words, the oxidation-reduction potential of the Al film is baser (in a negative side) than that of the ITO film, and in addition, the difference between the oxidation-reduction potentials of the Al and ITO films is large. Therefore, if both the ITO film and the Al film exist in the developer solution (which is an electrolytic solution), an electric current circuit is formed between the ITO film and the Al film by way of the developer solution. As a result, electrons emitted by the dissolution of the Al film will flow into the ITO film, thereby reducing and corroding the said ITO film.
Even if the TFT array substrate has a structure that all the surface of the ITO film is covered with the Al film or the photoresist film and therefore, the ITO film will not contact the developer solution, there is a possibility that the developer solution reaches the ITO film through pinholes or the like which are formed locally by the dissolution of the Al film or which have been present in the Al film itself. If so, an electric current circuit is formed in such a way that the Al film serves as a local anode and the ITO film serves as a local cathode. As a result, the dissolution reaction of the Al film and the reduction reaction of the ITO film make progress by using the difference of the oxidation-reduction potentials of the Al and ITO films as a driving force. In this way, the ITO film is corroded. This reaction is termed the “cell corrosion reaction” or “cell reaction”.
Moreover, there is a problem that stable electrical contact between the ITO film and the Al film is difficult to be generated. This is due to the following reason. Since the oxide-generating energy of Al is greater than that of In (indium), if the thermal hysteresis of 150° C. or higher occurs after depositing the Al film on the ITO film, or after depositing the ITO film on the Al film whose surface oxide has been removed, Al oxide having high electrical resistivity is likely to be generated on the contact interface of the ITO and Al films prior to the generation of In oxide having low electrical resistivity. As a result, the ITO film and the Al film are likely to be electrically insulated from each other.
It is known that these two problems can be solved by depositing an intermediate film (i.e., a barrier metal film) that suppresses the above-described cell corrosion reaction and that provides stable electrical contact between the Al film and the ITO film. As the intermediate film (the barrier metal film), a film made of molybdenum (Mo), chromium (Cr), or the like is preferably used.
In the above-described Patent Document 1, considering the possibility that the above-described cell corrosion reaction is not suppressed even if the intermediate film (barrier metal film) is deposited between the Al film and the ITO film, the following measure is suggested.
Specifically, after forming an Al film on an interlayer insulating film, the Al film is patterned to form the reflection electrode. Next, a photoresist film for the transmission electrode is formed on the reflection electrode and patterned. On the photoresist film thus patterned, an ITO film for the transmission electrode is formed. Thereafter, the patterned photoresist film is removed by the “lift-off” method and at the same time, the ITO film is patterned to form the transmission electrode (or the reflection electrode). With this method, almost all the Al film is covered with the photoresist film for the transmission electrode even after the patterning process of the Al film, where the remainder of the Al film is covered with the ITO film. Thus, the Al film does not contact the developer solution used for patterning the photoresist film for the transmission electrode. For this reason, the Al film and the ITO film are not simultaneously exposed to the developer solution, which means that disappearance of the Al film and blackening of the ITO film are prevented (see FIGS. 1 to 7, paragraphs 0049 to 0076).
The Patent Document 2 (Japanese Non-Examined Patent Publication No. 2005-266761) discloses a method of preventing the above-described cell corrosion reaction by using a metal film that raises the Al potential in the developer solution as an upper layer of the reflection electrode (Al film) (see FIGS. 1 to 3, paragraph 0027 to 0051), which is another measure.
With the method disclosed in the Patent Document 2, the transmission electrode formed by an ITO film is placed in the pixel region (i.e., in both the transmission region and the reflection region) of the semi-transmissive type LCD device. At the same time, in the reflection region, a reflection film (which contains at least Al) is formed on the transmission electrode (i.e., ITO film) and a metal film that raises the Al potential in the developer solution is formed on the reflection film. In the transmission region, the reflection film and the metal film do not exist and thus, the transmission electrode is exposed. As the metal film, a metal film that contains at least one of Ni, Fe, Pd, Pt, Rh, Re, Ru, Co, In, Nb, V, Mo, W, and Zr is used.
In the process of patterning the reflection film and the metal film, when the photoresist film formed on the metal film is selectively exposed to light and then, is developed with a developer solution, the metal film is placed on the reflection film. Therefore, even if Al is eluted in the developer solution, the potential of Al in the developer solution is kept higher than the corrosion potential of ITO. This means that even if the developer solution reaches the ITO film, the ITO film does not corrode. In this way, the reduction and corrosion of the ITO film due to the elution of Al in the developer solution is prevented (see FIGS. 1 to 3, paragraphs 0027 to 0042).
With the semi-transmissive type LCD device disclosed in the Patent Document 2, the reflection film may be made of Al alloy containing at least one of Ni, Fe, Pd, Pt, Rh, Ru and Co. In this case, the metal film can be cancelled (see paragraph 0050). In addition, a second metal film containing at least one of Cr, Ti, Ta and Mo may be formed below the reflection film (see paragraph 0051).
The Patent Document 3 (Japanese Non-Examined Patent Publication No. 10-173191) discloses a method of preventing the above-described cell corrosion reaction for the source electrode and the drain electrode, not the transmission electrode and the reflection electrode (see FIG. 1, paragraph 0018 to 0022).
With the method disclosed in the Patent Document 3, the source electrode and the drain electrode have a three-layer structure whose uppermost barrier layer is made of a material having a higher reduction potential in the developer solution than that of ITO. Thus, the ITO film for the pixel electrode (which is placed in a lower position of the three-layer structure) is not exposed to the developer solution in the development process of the photoresist film for patterning the three-layer structure. As a result, the above-described cell corrosion reaction is prevented.
Next, an example of the method of fabricating a TFT array substrate used in a prior-art semi-transmissive type LCD device with the structure that each pixel region is divided into the transmission region and the reflection region will be explained below with reference to FIGS. 1 to 11. This LCD device comprises the above-described intermediate film (i.e., the barrier metal film) between the Al film and the ITO film to suppresses the above-described cell corrosion reaction (see the Patent Document 1).
FIGS. 1 to 7 are partial plan views showing the structure of one pixel and its neighborhood of the said LCD device, respectively. FIG. 8 is a partial cross-sectional view along the line VIII-VIII in FIG. 7. FIGS. 9 to 11 are partial cross-sectional views similar to FIG. 8 showing the disconnection or breaking steps of the ITO film (i.e., the transmission electrode), respectively. As shown in FIG. 7, the pixel region P, which corresponds to one pixel, is divided into the transmission region T and the reflection region R.
First, as shown in FIG. 1, a glass plate 110 (see FIG. 8) is used as an insulative transparent substrate. On the glass plate 110, a metal film for gate electrodes 111a and scanning lines (gate lines) 111 is formed. For example, a film made of chromium (Cr), aluminum (Al), molybdenum (Mo), or titanium (Ti), or an alloy thereof is formed on the glass plate 110 and then, it is patterned by the photolithography and etching method, thereby forming the gate electrodes 111a and the scanning lines 111 which are united with each other. The scanning lines 111, which are extended along the horizontal direction of FIG. 1, are arranged along the vertical direction of FIG. 1 at equal intervals.
Next, a silicon nitride (SiNx) film serving as a gate insulating film 121 (see FIG. 8) is formed on the whole surface of the glass plate 110 by a CVD (Chemical Vapor Deposition) method, thereby covering the gate electrodes 111a and the scanning lines 111. Thereafter, a non-doped amorphous silicon (which may be termed “a-Si” later) film and a n-type amorphous silicon film doped with phosphorus (P) are formed in this order and then, these two a-Si films are patterned by the photolithography and etching method, thereby forming island-shaped semiconductor films 112 and 112a, as shown in FIG. 2. In the said pixel region P, the patterned semiconductor films 112 overlapped with the gate electrodes 111a are used as the active layers of TFTs. The patterned semiconductor films 112a overlapped with the scanning lines 111 are located at predetermined positions to be overlapped with signal lines (data lines) 113 which are formed later. The patterned semiconductor films 112a are provided to suppress the parasitic capacitance between the scanning lines 111 and the signal lines 113.
Next, a metal film similar to the metal film used for the scanning lines 111 is formed on the semiconductor films 112 and 112a and then, it is patterned by the photolithography and etching method, thereby forming the signal lines 113, source electrodes 113s, and drain electrodes 113d, as shown in FIG. 3. The source electrodes 113s and drain electrodes 113d, which are partially overlapped with the corresponding semiconductor films 112, constitute TFTs 125 along with the gate electrodes 111a and the gate insulating film 121. The drain electrodes 113d are unified with the corresponding signal lines 113. The signal lines 113 extending vertically in FIG. 3 are arranged horizontally at equal intervals in the same figure. The signal lines 113 and the scanning lines 111 constitute a matrix array.
Next, a passivation film 122 (see FIG. 8) is formed on the gate insulating film 121 over the whole surface of the glass plate 110, thereby covering the TFTs 125 and the signal lines 113. Thereafter, as shown in FIG. 4, contact holes 122a are formed in the passivation film 122 at the positions overlaid with the source electrodes 113s by an etching method. As the passivation film 122, a SiNx film is preferably used.
Next, an ITO film is formed on the passivation film 122 and patterned by the photolithography and etching method, thereby forming the transmission electrodes 114, as shown in FIG. 5. At this time, the transmission electrodes 114 are contacted with the corresponding source electrodes 113s by way of the corresponding contact holes 122a of the passivation film 122. In this way, the transmission electrodes 114 are electrically connected to the corresponding source electrodes 113s. 
In addition, the end of the transmission electrode 114 is overlapped with the scanning line 111 adjacent to the said pixel region P (which is located on the opposite side to the TFT 125 (on the upper side in FIG. 5)). However, the end of the said transmission electrode 114 is apart from the signal line 113 corresponding to the said pixel region P (which is located on the right side in FIG. 5) and from the signal line 113 adjacent to the said pixel region P (which is located on the left side in FIG. 5), and does not overlap with these two signal lines 113.
Next, a photosensitive organic resin film is formed on the passivation film 122 over the whole surface of the glass plate 110 and is selectively exposed to light and developed, thereby forming photosensitive interlayer insulating films 115 each having protrusions and depressions on its surface, as shown in FIG. 6. In the said pixel region P, the patterned photosensitive interlayer insulating film 115, which has an approximately rectangular planar pattern or shape, covers the whole TFT 125 and the part of the transmission electrode 114 adjacent to the TFT 125. The photosensitive interlayer insulating film 115 covers the part of the scanning line 111 (which is located on the lower side of FIG. 6) corresponding to the said pixel region P as well.
Next, a metal film (for example, which is made of Mo or an alloy of Mo) for the barrier metal film 123 is formed on the passivation film 122 over the whole surface of the glass plate 110. On the metal film thus formed, another metal film (for example, which is made of Al or an alloy of Al) for the reflection electrode 116 is formed over the whole surface of the glass plate 110. Then, after a photoresist film 124 with a predetermined pattern is formed on the metal film for the reflection electrode 116, these two metal films are patterned with the photoresist film 124 as a mask, thereby forming selectively the barrier metal films 123 and the reflection electrodes 116, as shown in FIGS. 7 and 8.
The barrier metal films 123 and the reflection electrodes 116 have planar patterns or shape which are approximately the same as the patterned photosensitive interlayer insulating films 115 and which are slightly smaller in size than the same films 115, respectively. In the pixel region P, the end of the barrier metal film 123, which is close to the transmission electrode 114, is in contact with the transmission electrode 114. Due to this contact, the barrier metal film 123 and the transmission electrode 114 are electrically interconnected. Moreover, the reflection electrode 116, which is formed on the barrier metal film 123, is in contact with the barrier metal film 123. Thus, the reflection electrode 116 is electrically connected to the transmission electrode 114 by way of the barrier metal film 123.
In the above-described way, the TFT array substrate used for the prior-art semi-transmissive type LCD device is completed. The TFT array substrate thus fabricated is joined to an opposite substrate (not shown) and then, a liquid crystal layer is placed in the gap between these two substrates and sealed, resulting in a liquid-crystal panel (or a LCD panel). Furthermore, a backlight unit is built in the panel. In this way, the prior-art semi-transmissive type LCD device is fabricated.
According to the inventor's research, the above-described fabrication method has the following problem.
Specifically, the processes of forming the barrier metal film 123 and the reflection electrode 116 are carried out in such a manner as shown in FIGS. 9 to 11. In these processes, it is usual that the thickness of the barrier metal film 123 is set at a considerably large value, for example, approximately 0.2 μm to 0.4 μm. This is to make the distance between the reflection electrode 116 formed by a metal film (e.g., an Al or Al alloy film) and the transmission electrode 114 formed by an ITO film as much as possible, thereby lowering the possibility of the cell reaction. For this reason, when a metal film 123a (e.g., a Mo or Mo alloy film) for the barrier metal film 123 and a metal film 116a for the reflection electrode 116 (e.g., an Al or Al alloy film) are successively formed on the passivation film 122, cracks 130 are likely to occur in the metal film 123a in the vicinity of the scanning line 111, as shown in FIG. 9. These cracks 130 may occur not only in the metal film 123a but also in the underlying transmission electrode 114, the passivation film 122, and the gate insulating film 121.
When the metal films 123a and 116a are patterned, a photoresist film 124 is formed on the metal film 116a and is selectively exposed to light using an appropriate mask. Then, the photoresist film 124 thus exposed is dipped in a developer solution with high alkalinity or is contacted with the said developer solution by falling it like a shower over the photoresist film 24, thereby selectively removing the exposed portions of the film 124. Thus, the state as shown in FIG. 10 is obtained. At this time, although the meal film 116a made of Al or Al alloy is in contact with the developer solution, the transmission electrode 114 made of ITO ought not to be in contact with the said developer solution. In fact, however, due to the existence of the cracks 130, the said developer solution reaches the transmission electrode 114 also and thus, a cell circuit is formed by the metal film 116a, the transmission electrode 114, and the developer solution. Accordingly, the transmission electrode 114 is reduced and corroded and as a result, a disappeared portion 140 is formed in the transmission electrode 114 in the neighborhood of the scanning line 111.
Therefore, when the reflection electrode 116 and the barrier metal film 123 are formed by selectively etching the metal films 123a and 116a using the photoresist film 124 as the mask, the transmission electrode 114 has the disappeared portion 140. If the disappeared portion 140 becomes large, the transmission electrode 114 is separated from the barrier metal film 123, which leads to failed pixels termed the “point defects”. Even if the transmission electrode 114 is not separated from the barrier metal film 123 due to the disappeared portion 140, the optical transmittance property of the transmission electrode 114 degrades. Since the occurrence of the cracks 130 is unable to be avoided in the present circumstances, it is necessary to prevent the formation of the disappeared portion 140 with some measure.
Explanation is made so far for the scanning line 111. However, the transmission electrode 114 is placed close to the signal line 113 in the said pixel region P. Accordingly, a similar problem will occur for the signal line 113.
The cell corrosion reaction can be prevented by the methods disclosed in the above-described Patent Documents 1 to 3. However, any of these methods has a disadvantage that the fabrication processes need to be changed significantly.
In addition, the above-described explanation about the transmission electrodes is applicable to the common electrode of the IPS (In-Plane Switching) type LCD device.